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1994 Chevy suburban 6.5TD

S

seza

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Location
England
Hi All,
My 1994 Chevy sub 6.5TD with Auto transmission sometimes hesitates while driving at high speed, anything over 40, 50 or more mph.
While my foot is still on the accelerator (with no change) the truck slows down and and rev counter goes mad momentarily then it continues as normal,
I will be grateful for any ideas as to what is causing it?

many thanks
 
Fuel delivery/LP or a PMD. Possible ground problem.
Sounds like a dirty filter. Do you know what fuel pressure you have?
 
Thank you for your reply,
As far as I can see it has a mechanical pump therefore no PMD,
I am not sure about fuel pressure.
you may be right about fuel filter as I have not changed that for a while.
 
rpm goes mad??? rpm signal comes from alternator. May be alternator on the way out.
Something on the belt system is not right.

Agree with Burning Oil about filter. Make sure the OPS is good since in a 94, it runs the LP.
 
My first thought was PMD. Normally a 94 would have an electronic IP and PMD. It could have been changed to a 93 or earlier IP. Does yours have a cable connecting the throttle pedal to the IP?

Don
 
Thanks guys,
Yes Don - it does, it has two cables, if mem serves me right one goes to throttle pedal the other goes to to a box just in front of the driver,
JMJNet, RPM reading seems to be very erratic most of the time even if the car is NOT hesitating, alt seems to be charging ok but the belt is slightly wearing where it meets the AC motor, there is fine rubber dust accumulation there,
the OPS? I know I bought a spare just incase but how do I get to it? is it easy to change?
 
OPS can be easy if it is behind the head. Look at the back side of the driverside head, pointing toward the engine valley and if it is the original engine the OPS is likely there. I cant remember what size socket (I have an OPS specific socket) to use, I think auto zone would have the OPS socket, if not a DEEP socket of the right size will work. You will need about 8 inches worth of extensions and a U Joint like attachment for the socket.

Changing filters is always a good start.

Then you need to check the LP is operating by opening the filter drain valve while the engine is at idle. There should be fuel flowing on the ground and the engine shouldn't die. If it dies either the LP is shot or the OPS is bad.

Checking to be sure the LP is good is as easy as hooking alligator clamps (jumped to 12V/battery) to the LP termminals.

If the filter is clean and LP test checks out then it is likely the fuel shut off solenoid (FSO) shorting out and you need to check relays and wiring, GROUNDS are always a good thing to check, or you might have an IP that is going bad. Check the cheap and easy stuff first!
 
Pull the codes out of the "transmission" computer. Standard ECM but it only controls the transmission so i call it a transmission computer. You can do this with a paperclip. All the computer does is control the transmission and because it isn't emissions related it will never turn on the service engine soon light for any faults it has. It sounds like the transmission is dropping out of overdrive or slipping allowing you to slow down and the engine run away. How does the transmission oil look?
 
Thank you guys,
Burning Oil: the engine was taken from another 94 truck, may be it was an earlier model am not sure, the chevy is def a 1994
WarWagon, good point about the trans oil, it is as new as it was put in first time, it is red no sign of wear despite it being there for a long time (over two yrs),
also the oil level is bit confusing: the cold reading indicate its way above recommended level and so is the hot - so am not sure it has too much trans oil.
 
94's did not come factory with a DB2 Mechanical pump. At some point the truck or suburban was converted. If it was improperly converted then that might be why tranny is acting weird.

Just to be clear it is a GMT 400 suburban. 93 diesel suburban would be a square body NA 6.2 IF they made one in 93.
 
A picture of the underhood area with the cable to the IP would be useful. Sounds like it was converted as they did not make a new body style DB2 suburban. At least in the USA.

The pump numbers of the IP would be very useful. Depending on what pump was used this could be your problem. The 1986 model year 6.2 pump will work on a 6.5, however, the throttle shaft does not rotate as much as the 6.5 IP does. This seriously messes up the computer by telling it you are not at WOT when you are. this causes less pressure to be used and may slip the transmission esp. under high load.

Your readings indicate an overfull transmission. this will foam the transmission fluid and more or less FUBAR it. You can not compress air and shifting and holding gears become an issue. Radiator tanks are famous for pinhole leaks dumping coolant into transmissions. Rust on the top of the dipstick is a indicator of excess moisture in the transmission. Your fluid level needs to be brought down. Suck it out via the dipstick tube with an oil sample line, loosen and leak it out of a transmission cooler line or change the fluid and filter.

The proper fluid level should solve your problem. Cole readings are useless. Use the hot reading always.

I highly recommend you test the radiator for pinhole leaks in the transmission cooler if you can not determine if anyone overfilled the transmission.

Also the transfer case can leak back and forth between the transmission and transfer case. Usually it is the transfer case that gets overfilled from the oil seal between then leaking. Check it in case it is overfull of oil and leaking back into the transmission.
 
Gents,
The fuel filter was bad esp after I've used biodiesel recently, so I hope changing it will solve my problem,
although the oil level in the trans is worrying - as one of you suggested I am going to try reduce it via the dipstick.
 
Gent,
I have some updates about my Chevy

1. the original problem with hesitation is now FIXED - thanks to one of you who suggested to check the OPS,
all I did was to press the wiring coming out of it down bit and it worked, the rev counter is now smouth with hesitation at all

2. The issue with Transmission oil level, I check it again today, it seems to froth up fair bit because it shows on the dip stick at about 1/2in above max hot level
as froth.

3. still with oil, I have a persistant oil drip from the hole in the sum between the engine sum and trans, not lot, overnight it drips about two spoon full,
it is black, considering that the rest of engine is very clean, where is it comning from?

4. Finally - from the pictures attached could you identify the transmission box - please

many thanks

engine view dr.jpgengine view pa.jpgIP.jpgtrans-engine.jpg
 
Trans is a 4L80E
You definately have a DB2 IP. Looks like a home made throttle bracket.
Can you tell me more about the device on the intake area on passenger side. In your picture its right above the CDR. Where are the wires leading to? What do you think it is?
 
Also, I like your oil filler cap. Can you get a close up of it. Thanks


Edit: just noticed you have a 1996&^ serp belt system.
 
thank you for your input - B. Oil
the 4L80E and the DB2 - are they any good? anything I should watch out for?

as for the the small black box near the CDR - your geuss is as good as mine!,
when I first got the vehicle I thought it might have been the PMD
then I realised I had a mechanical IP, I can try to trace the wiring tmrw and let you know if that helps.
 
4L80E is about the best trans you can have IMO. Same with DB2.

Yea, any info on that thing would be good. Looks like it might be part of the conversion to me.
 
Great,
I will check the wiring and also take a close up pic of the oil cap and send them tmrw.
what do you think about the oil leak? is it engine oil?
talking of oil, can I use synthetic oil in the engine?

Little info I have about this chevy is that it has been imported into the UK in 2004 and then in 2009/10
the previous owner converted it from gas to diesel, the engine was taken from a 1996 truck model K30943 [VIN 1GTHK33F5VTF009115].

Apart from fuel consumption I love the vehicle, it is the best I have driven, very comfy, easy to work on, build like a rock, very American ... big n strong!
 
The oil leak is most likely engine oil from the rear crank seal.

You can use synthetic oil.
 
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